The Aliso Canyon wellhead that blew out in October | Photo: Earthworks/Flickr/Creative Commons License
Watch SoCal Connecteds special report on Porter Ranch Wednesday, March 2 at 8:00 p.m.
As of February 18, the massive natural gas leak at the Aliso Canyon oil and gas field in the Santa Susana Mountains is declared plugged. Residents of Porter Ranch, the community hardest hit by the catastrophic fossil fuel spill, are preparing to return to their homes, and the states lawmakers are now deciding whether to make SoCalGas, the owner of the leaking well, close down its Aliso Canyon gas storage operations entirely.
The announcement that the leak had been plugged didnt come a moment too soon. The Aliso Canyon leak has been ranked as an environmental disaster rivaling the 2010 Deepwater Canyon oil spill in the Gulf of Mexico in terms of its eventual damage to the environment. Thats mainly due to the damage the spilled natural gas will cause to the planets climate: methane, the main constituent of natural gas, is a potent greenhouse gas, about 21 times more damaging to the climate than CO2 (carbon dioxide) for 100 years after its released into the atmosphere. The spill at Aliso Canyon -- the largest single release of methane in U.S. history -- was a significant setback to Californias attempts to address the threat of climate change.
So were rightly relieved that the leak seems to be over. But before you exhale all the way, there are a few things to keep in mind about the larger context in which the Aliso Canyon disaster happened.
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What does the gas leak near Porter Ranch say about our energy future?
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SoCal Connected
Special Report: Porter Ranch Gas Leak Aftermath On Oct. 23, 2015, workers at a SoCalGas facility in the hills north of Los Angeles detected a leak.
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SoCal Connected
Special Report: Porter Ranch Gas Leak Pt. 1 On Oct. 23, 2015, workers at a SoCalGas facility in the hills north of Los Angeles noticed a strong odor of methane gas.
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SoCalGas Answers Questions About Gas Leak For more than 100 days, natural gas spewed uncontrollably from a well at the Aliso Canyon Natural Gas Storage Facility.
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SoCal Connected
Timeline: Porter Ranch Gas Leak Timeline: Porter Ranch Gas Leak Timeline of the major events surrounding the Porter Ranch gas leak.
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The only thing unusual about the Aliso Canyon gas leak was its size.
Aliso Canyon may have been the largest single atmospheric release of methane in U.S. history, but despite its devastating scale, methane releases continue at a rate that might well make Aliso Canyon seem like a blip. Preliminary estimates of the total amount of gas leaked at Aliso Canyon run at least 80,000 total metric tons over the 118-day course of the leak. Thats about 29 metric tons of methane spilled per hour, on average. (Some estimates run as high as 100,000 metric tons.)
Sounds like a lot, and it is. But consider this: according to the U.S. Environmental Protection Agency, the American oil and gas industry spills about 7.3 million metric tons of methane a year into the atmosphere. Thats an average of more than 833 metric tons of methane per hour, almost 30 times the conservatively estimated hourly rate of the Aliso Canyon leak.
Put another way: The typical background rate of ongoing methane emissions just from the oil and gas industry equals about 30 Aliso Canyons.
Some of that leaks from aging pipelines and storage facilities, oil refineries and other infrastructure. Some of it leaks from the gas fields from which gas and oil is extracted in the first place. The EPA once estimated that 2 percent of the methane produced at many gas fields escaped into the atmosphere. A recent study of gas fields in Colorados Denver-Julesburg basin put the actual figure twice as high, and so did a December 2015 study of gas fields in north Texas.
And thats just direct emissions by the fossil fuel industry. Methane also leaks from retail gas companies aging infrastructure -- as residents of San Bruno tragically learned when a poorly maintained PG&E pipeline exploded in 2010. As Christie Aschwanden reported this month at fivethirtyeight.com, major cities often have hundreds or thousands of natural gas leaks going unreported and unrepaired. More gas is spilled by agriculture, deliberate burning of biomass, and by garbage decomposing in landfills. All in all, the EPA estimates that the U.S. dumps 25 million metric tons of methane into the atmosphere -- 312 Aliso Canyons -- each year. And other sources suggest its closer to 40 million tons per year.
Most of the gas leaked from Aliso Canyon came from southwestern oil and gas fields where hydraulic fracturing, and methane leakage, is commonplace.
Despite Californias role as a pioneer in fossil fuel extraction, with oil and gas wells dating to the early 20th Century, the state actually relies on supplies from other parts of the country for the large majority of its natural gas use. SoCalGas, which operates the Aliso Canyon well that caused the leak, imports the majority of its natural gas via the El Paso Natural Gas and Transwestern pipelines from gas fields in the American Southwest.
That bright red dot is the San Juan Basins methane leak. Map: NASA
And that mainly means SoCalGas filled the Aliso Canyon storage reservoir with gas from fields in the San Juan basin in the Four Corners area, which has been the nations largest single source of methane emissions for years. So much methane leaks into the atmosphere from the San Juan Basins 40,000-plus wells that one basin contributes 10 percent of the natural gas industrys methane emissions.
The San Juan Basin was a major emitter of methane even before the advent of the now-widespread practice of hydraulic fracturing, a.k.a. fracking. But over the last few ye
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The Aliso Canyon wellhead that blew out in October | Photo: Earthworks/Flickr/Creative Commons License 
